1. Field of the Invention
This invention relates to a method and an apparatus for recording and/or reproducing information signals for a plurality of sorts of optical discs having reciprocally different values of track pitch of the recording tracks and hence differential recording densities. This invention also relates to a method for adjusting the aberration of the present optical disc recording and/or reproducing method and apparatus.
2. Description of Related Art
In an optical disc used up to now as a recording medium for information signals, attempts have been made for raising the recording density. For example, in the case of a magneto-optical disc having a diameter of approximately 65 mm, proposals have been made for reducing the track pitch of the recording track carrying the information signals from 1.6 μm to 0.95 μm to increase the recording density by a factor of approximately five.
For recording information signals on the magneto-optical disc, having the reduced track pitch as described above, and for reproducing the recording information signals, the spot diameter of the light beam scanning the recording track formed on the magneto-optical disc needs to be of a smaller value. The reason is that, if the spot diameter of the light beam becomes larger than the track pitch of the recording track, the recording track cannot be tracked correctly, with the result that information signals cannot be recorded or reproduced on or from a desired recording track.
Thus, proposals have also been made for using an optical pickup device provided with a light source designed for radiating a light beam of a shorter wavelength for reducing the spot size of the light beam illuminated on the magneto-optical disc. If the optical pickup device, radiating the light beam of the short wavelength, is used for recording and/or reproducing a magneto-optical disc, the desired recording track cannot be tracked correctly, because the spot diameter of the light beam is markedly smaller than the recording track width, with the result that the information signals cannot be recorded or reproduced correctly.
For differentiating the track pitch values of the recording tracks from one another for ultimately enabling recording and/or reproduction of plural sorts of the magneto-optical discs with different recording densities by a common optical disc recording and/or reproducing apparatus, there has also been proposed an optical disc recording and/or reproducing apparatus including an optical pickup device having a plural number of light sources each radiating a light beam of a shorter wavelength and a light beam of a longer wavelength.
This optical disc recording and/or reproducing apparatus is designed to switch between the plural light sources to radiate light beams of different wavelengths to adapt itself to a plural number of sorts of the magneto-optical discs having reciprocally different values of the track pitch of the recording tracks.
With the magneto-optical disc having the track pitch of 1.6 μm, double refraction is more significant than with a magneto-optical disc having a track pitch of 0.95 μm, and hence astigmatic aberration is produced in an optical system when the light beam is transmitted along an optical path in the magneto-optical disc. So, in the optical disc recording and/or reproducing apparatus, dedicated to a magneto-optical disc having a track pitch of 1.6 μm, the value of this astigmatic aberration is supervised with respect to the entire optical system. On the other hand, with the magneto-optical disc having the track pitch of 0.95 μm, the double refraction is lesser than with the magneto-optical disc having a track pitch of 1.6 μm, such that the astigmatic aberration is suppressed in the optical system
[Problems to be Solved by the Invention]
With the above-described optical pickup device, having a plural number of the light sources, the device itself is increased in size to render it difficult to use the device in an optical disc recording and/or reproducing apparatus required to be reduced in size.
With the semiconductor laser, radiating a light beam of a shorter wavelength, such as approximately 650 nm, power consumption is larger than with the semiconductor laser radiating a light beam of a longer wavelength, such as approximately 780 nm. The optical pickup device, employing a semiconductor laser with a larger power consumption, is not suited to a battery-powered portable optical disc recording and/or reproducing apparatus. Additionally, with the semiconductor laser with large power consumption, the temperature coefficient is larger and undergoes significant self-heating, so that, when the semiconductor laser is mounted on an optical pickup device, measures against heat radiation need to be taken to cause stabilized oscillations of the light beam when the semiconductor laser is mounted on the optical pickup device, with the result that it becomes difficult to reduce the size and the thickness of the optical pickup device.
Moreover, the semiconductor laser radiating a light beam of a shorter wavelength is more expensive than the semiconductor laser adapted for oscillating the light beam with a wavelength of 780 nm, which has so far been used extensively, with the result that, if this semiconductor laser is used, it is not possible to reduce the cost of the optical pickup device and hence that of the recording and/or reproducing apparatus.
If, with the use of the optical disc recording and/or reproducing apparatus, dedicated to a magneto-optical disc having a track pitch of 0.95 μm, information signals are recorded or reproduced on or from a magneto-optical disc having a track pitch of 1.6 μm, there is raised a problem that astigmatic aberration is liable to be produced in the recording and/or reproducing optical system.
If, in a recording and/or reproducing apparatus dedicated to a magneto-optical disc having a track pitch of 0.95 μm, information signals are to be recorded or reproduced on or from a magneto-optical disc having the track pitch of 1.6 μm, there is raised a problem that a difference is produced, under the effect of the aforementioned astigmatic aberration, between an optimum point of the focusing bias in detecting the ADER (address in progressive error rate), as an error rate of the ADIP (address in pregroove), and an optimum point of the focusing bias of the RF signals of the magneto-optical disc having the track pitch of 0.95 μm.
It is specifically necessary to electrically offset the focusing bias, optimized for recording and/or reproduction of information signals for a magneto-optical disc having a track pitch of 1.6 μm, with respect to the focusing bias of the optical disc recording and/or reproducing apparatus optimized for recording and/or reproducing information signals on or from a magneto-optical disc having a track pitch of 0.95 μm.
However, the optimum offset undergoes variations, due to the astigmatic aberration present in the individual recording and/or reproducing optical systems, to render offset adjustment difficult.
There are also occasions wherein, depending on changes in shape of the magneto-optical disc or to tilt of the magneto-optical disc, the light beam is not incident at right angles to the recording surface of the magneto-optical disc. In this case, there is raised a problem that the light beam incident on the recording surface of the magneto-optical disc is not reflected in a direction perpendicular to the recording surface of the disc, such that the reflected light beam suffers coma aberration to deteriorate the readout efficiency of the information signals.
It is therefore an object of the present invention to provide a recording and/or reproducing apparatus in which the apparatus may be further reduced in size and thickness and in which information signals can be recorded and/or reproduced on or from a magneto-optical disc having a track pitch of 0.95 μm and a magneto-optical disc having a track pitch of 1.6 μm. It is another object of the present invention to provide a method for adjusting the aberration for such recording and/or reproducing apparatus